In general, this invention relates to a device for transferring an acoustic signal. More specifically, this invention relates to a device for transferring an acoustic signal produced by a telephone handset during a performance test.
Throughout the environment, radio frequency (RF) signals are present at various strengths. A consumer product will occasionally demodulate these RF signals unintentionally. When the consumer product is a telephone, this unintentional demodulation can cause annoying RF interference, which is defined as performance degradation, malfunction, or equipment failure due to the presence of RF signals.
Telephone designers address the problem of RF interference by attempting to minimize the vulnerability of their product to extraneous radio signals. As telephone designer's efforts in this area have progressed, certain performance criteria have developed, such as the ANSI/TIA/EIA 631 standard. RF immunity is defined as the ability of equipment to meet the performance criteria specified in the presence of RF signals. According to the ANSI/TIA/EIA 631 standard, a telephone has RF immunity if the demodulated acoustic output from the telephone handset does not exceed 55 dBSPL, except in the frequency band from 500 kHz to 2 MHz where the demodulated acoustic output shall not exceed 45 dBSPL, when the telephone is subjected to a radiated electric field having a frequency of 150 kHz to 150 MHz, an unmodulated field strength of 3 VRMS/m, and the modulation of a 1 kHz sinusoidal wave with an 80% amplitude modulation.
Generally, to determine whether a telephone has RF immunity, the telephone is isolated from outside RF signal sources, subjected to an RF field, and the acoustic output from the telephone handset is measured, plotted and displayed. One suggested system and method for conducting such an RF immunity test includes placing the telephone inside an RF chamber, placing a microphone in proximity to the transmitter portion of the telephone handset, and securing the chamber. A signal generator is then used to produce an RF signal that is broadcast on an antenna inside the RF chamber. The microphone detects the acoustic output from the telephone handset, if any, and converts it to an electrical signal that is transferred over a signal lead to a monitoring apparatus.
When the specific system and method described above were used to perform an RF immunity test, the monitoring apparatus displayed an RF interference pattern consistent with the pattern shown in FIG. 6. Clearly, the pattern shown in FIG. 6 has many measurements above 55 dBSPL. However, after analyzing the results and the system and method used to achieve the results, it was determined that a significant portion of the recorded RF interference did not derive from the telephone being tested, but instead was a result of the presence of the microphone inside the RF chamber. Thus, the test system must be changed, altered or modified so that the pattern recorded more accurately reflects the RF interference, if any, of the telephone being tested.